Various techniques to achieve directional hearing in a hearing aid have been suggested over the years. Examples of such techniques are as follows:
Matched pair of two omni-directional microphones: Directional hearing in hearing aids may be achieved by the use of a matched pair of two omni-directional microphones. To generate a directional output signal the signals from the omni-directional microphones are subtracted. An electrical time delay may be applied to one of the signals to shift the notch angle of the polar pattern. It is a disadvantage of the matched pair that in case of a mismatch/drift the directivity degrades heavily, in particular in the low frequency ranges. Moreover, matching microphones as well as amplitude/phase correction in the hearing aid production are time consuming manual operations.
Analogue directional microphone: Directional hearing in a hearing aid may also be achieved by the use of an analogue directional microphone. An analogue directional microphone is a microphone with one sound port in the front and one sound port in the rear volume. The advantage of an analogue directional microphone is that directionality cannot be degraded by drift or mismatch. However, the notch angle is at a fixed position and cannot be shifted by processing for beam forming purposes.
WO 2012/139230 discloses PU microphone module consisting of one omni-directional microphone (P) and one analogue directional microphone (U). The microphone module has two ports. The directional microphone picks up the pressure difference between front and rear port. In one embodiment the omni-directional microphone picks up the pressure at the front port of the module. Another embodiment is that the omni-directional microphone picks up the average of the pressures at front and rear port. The advantage of the PU microphone module is that the directional output is robust to compensate for mismatch/drift because it makes use of an analogue directional microphone which has a stable notch at 90 degree. The closer the desired notch angle is to 90 degree the smaller the impact of mismatch/drift on directionality. However, for notch angles close to 180 degree mismatch/drift still have a significant impact on directionality.
The so-called Jacobian module, cf. for example U.S. Pat. No. 8,254,609 comprises two directional microphones and one omni-directional microphone. The main advantage of the Jacobian principle is that a higher order directionality can be obtained. However, it is a disadvantage that the two directional microphones need to be matched very tightly. In case of mismatch/drift the directivity of the module degrades heavily.
Finally, the Blumlein pair is a stereo recording technique (also known as M/S technique) that makes use of two directional microphones. One of the directional microphones has a cardioid polar pattern (notch at 180 degree) and the other one is a dipole (notch at 90 degree). The microphones are oriented in a 90 degree angle towards each other. It is disadvantage of the Blumlein pair that it is a rather bulky design that requires a significant amount of space.
U.S. Pat. No. 5,473,701 teaches a method of enhancing the signal-to-noise ratio of a microphone array with an adjustable polar pattern by signal processing means. For illustrative purposes, back-to-back cardioid sensors are applied in U.S. Pat. No. 5,473,701. The back-to-back cardioid sensors are obtained from a differential arrangement of two omni-directional microphones. The signal processing suggested in U.S. Pat. No. 5,473,701 is also applicable in relation to sensors of other back-to-back polar patterns than cardioids.
EP 2 107 823 A2 shows a microphone module comprising a first and a second directional microphone. According to paragraph [0029] of D1, an acoustical input port is provided for an omni-directional microphone 601 and a directional microphone 603, cf. FIG. 6 of EP 2 107 823 A2. Thus, there is in EP 2 107 823 A2 no disclosure of a middle sound inlet arrangement being acoustically connected to a front and a rear membrane of respective directional microphones.
EP 2 723 102 A2 teaches in relation to FIG. 4 and paragraph [0083] that a sound filtering element 60′ can be used to divide a common rear volume into two separate rear volumes each having a membrane in acoustical connection thereto. Thus, there is in EP 2 723 102 A2 no disclosure of a sound inlet having an acoustical resistance inserted therein.
It may be seen as an object of embodiments of the present invention to provide a microphone module comprising first and second directional microphones having back-to-back polar patterns.
It may be seen as a further object of embodiments of the present invention to provide a microphone module comprising a first directional microphone having a cardioid polar pattern as well as a second directional microphone having an anti-cardioid polar pattern.